A typical inkjet printer uses one or more printheads to form an ink image on an image receiving surface. Each printhead typically contains an array of individual inkjets for ejecting drops of ink across an open gap to the image receiving surface to form an ink image. The image receiving surface may be on a continuous web of recording media, a series of media sheets, or on a rotating image receiving member, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating member and a transfix roller.
Modern inkjet printers typically receive digital image data in a variety of formats and form ink images on a print medium that reproduce the original digital image. One challenge in forming printed images includes the accurate reproduction of colors in a physically printed image from the digital image data. For example, the digital image data are often encoded in a format that is unsuitable for direct operation of inkjets in the printer. Instead, the digital image data are encoded in either a device-independent color space, such as L*a*b*, or are encoded in a digital format such as the red, green, blue (RGB) that is associated with display screens rather than printed images. The printer or an intermediate computing device converts the digital image data into a data format that corresponds to the ink colors that are available in the printer. One type of printer forms images using cyan, magenta, yellow, and black (CMYK) inks. The printer reproduces a wide range of colors by interspersing small drops of the basic CMYK colors on the print medium in halftone or dithered ink drop patterns. The human eye perceives different colors from the combination of the CMYK colors. Existing standards, which include profiles from the International Color Consortium (ICC), are used to covert image data from an input color space into a color space that is used to eject ink drops from the inkjets to form the printed images.
The image data for many printed images specify one or more “spot colors.” As used herein, the term “spot color” refers to a specific color that is reproduced in a printed image with high accuracy. While a color printer generates color reproduction for a wide range of colors, the accurate reproduction of a spot color is often very important to the perceived quality of the printed document. In some printers, specific combinations of ink colors such as CMYK inks can reproduce the spot colors.
In many printer configurations, the printer ejects ink drops to form a printed image in a “colorant-limited” operating mode. In an inkjet printer, the operated mode is referred to as an “ink-limited” operating mode, while in a xerographic printer the mode may be referred as a “toner-limited” operating mode. That is to say, the printer forms printed images using a reduced amount of colorant compared to the maximum operating limits of the printer. Examples of ink limiting include limiting the proportion of the image receiving surface that can be covered with ink in a predetermined pattern to reproduce a color or to limit the total mass of ink that is printed in a predetermined location to reproduce the color. Printers operate in the ink-limited modes to, for example, reduce the total amount of ink that is consumed during a print job, prevent ink oversaturation of a print medium such as paper, and prevent excess ink from offsetting from the print medium to the components in the printer. The printer uses an ink-limited color profile to convert color coordinates in the device independent color space to a gamut of CMYK ink patterns that can be printed on the medium without exceeding the ink consumption limits.
In inkjet printers, ink-limited operating modes are unable to reproduce some spot colors. More generally, in color printers that use ink, toner, or other colorant materials, the colorant-limited operating modes can prevent the reproduction of some spot colors. For example, if a spot color for red requires a density of magenta and yellow inks that exceed the maximum ink density in the ink-limited profile, then the printer approximates the spot color using reduced amounts of the magenta and yellow inks. While the approximated colors in a colorant-limited gamut are acceptable for many printed images, the printer produces unacceptably inaccurate spot color approximates in the colorant-limited printing mode. Consequently, improvements to the operation of printers, including but not limited to inkjet printers, to generate printed images using colorant-limited printer modes while also reproducing spot colors with improved accuracy would be beneficial.